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525A
Users Manual
3-8
Default RTD Coefficients
The USR_DEF function of the calibrator uses the Calendar-Van Dusen equation
for outputting and measuring custom RTDs. The C coefficient is only used for the
subrange –260 to 0 degrees Celsius. Only the A and B coefficients are needed for
the subrange 0 to 630 degrees. The R0 is the resistance of the probe at 0 degrees
Celsius. All of the USR_DEF will be set to PT385 as shown in Table 2-1..
Table 3-1. Default RTD Coefficients
Sub range
R0
Coefficient A
Coefficient B
Coefficient C
USR_DEF1
0 to 630
100
3.908X10-3
-5.8019X10-7
USR_DEF2
-260 to 0
100
3.908X10-3
-5.8019X10-7
-5.8019X10-12
USR_DEF3
0 to 630
100
3.908X10-3
-5.8019X10-7
USR_DEF4
-260 to 0
100
3.908X10-3
-5.8019X10-7
-5.8019X10-12
USR_DEF5
0 to 630
100
3.908X10-3
-5.8019X10-7
If other types of RTD are required the table below shows the coefficients for
PT391 and PT392. Again C is only used for temperatures below 0 Celsius.
Table 3-2. Other Commonly Used RTDs
RTD Type
Coefficient A
Coefficient B
Coefficient C
PT392
3.9848X10-3
-5.87X10-7
-4X10-12
PT391
3.9692X10-3
-5.8495X10-7
-4.2325X10-12
The SPRT 25 ohm function of the calibrator uses ITS-90 standard coefficients for
measuring a SPRT. Since the five coefficients are deviations all of them will be set
to 0.
Note
The measuring function of the calibrator is limited to 100 ohms, so it
can not be used for >100 ohm SPRT.
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